Various pyrotechnic, electronic, and mechanical delays that control the time to function of propellant, pyrotechnic, and explosive devices are known in the art. Pyrotechnic delays are frequently used to control functions of munitions, such as self-destruct and self-disable, and the propellant ignition time of a rocket or rocket assisted projectile, where the timing of the ignition of the propellant is critical in achieving maximum range. Pyrotechnic delays generally rely on the controlled burning of a pyrotechnic material, acting essentially as a fuse, such that the length of the column of pyrotechnic material and the burning rate of the material determine the time of the delay. That is, the delay time is the time between the ignition of the pyrotechnic column and the ignition of the propellant, pyrotechnic, and explosive device by the heat and/or flame output generated by the combustion of the pyrotechnic column. For example, in a projectile having a range extending propellant, the initial end of a pyrotechnic delay column/ignition train is ignited as the shell is fired. The range extending propellant grain is then ignited by the heat and/or flame output of the pyrotechnic delay column/ignition train when the burning portion of the delay column/ignition train reaches the propellant. The delay time is then the time between the ignition of the pyrotechnic delay column/ignition train and the ignition of the range extending propellant grain by the output of the pyrotechnic delay column/ignition train.
Pyrotechnic delays typically require a rapid burn rate for reliability. Slower burning pyrotechnics are harder to ignite than fast burning pyrotechnics, and, typically, do not burn at a constant rate. Therefore, the delay time of slow burning pyrotechnics is less reliable than faster burning pyrotechnic delays, and reliable longer delay times are not easily obtained.
Control of the delay time of reliable, fast burning pyrotechnic delays is achieved by determining the burn rate of the pyrotechnic material and the length of pyrotechnic material that is needed to burn for the required time. As a result, the use of pyrotechnic delays in timing munition events is primarily limited by the space requirements of the munition, i.e., by the length of the column that will fit in the munition. Therefore, extended delay times are difficult to achieve because of the excessive length of pyrotechnic material required and/or the need for a slow burning pyrotechnic material. Typical size limitations for pyrotechnic delays using bum rate and column length to control the delay are driven by a nominal lower bum rate of about 0.1 inch (2.5 mm) per second for pyrotechnic columns having a cross section of about 1/8 inch (3 mm) for columns up to about 3/8 inch (9.1 mm), with cross-sections of about 1/4 inch (6.4 mm) for longer columns. The burn rate, heat loss, and column cross section are all closely interrelated, and, thus, the column must be carefully tailored to obtain reliable performance at or near the limits described above.
Electronic delays are typically used in situations where pyrotechnic delays are inadequate. The requirements for the self-destruction of munitions dictates long delay times, i.e., in excess of 30 seconds. For long delay times, electronic delay mechanisms are typically utilized because pyrotechnics cannot provide the delay time required within the packaging constraints. For time delays greater than 30 seconds, electronic delays offer greater packaging efficiency than pyrotechnic delays, but at a significant cost premium. In addition, electronic delays are much less durable than pyrotechnic delays, being comparatively fragile and, thus, susceptible to damage by the high acceleration or "g" loading experienced when the projectile is fired or the munition is launched or ejected.
Mechanical delays are less common for timing munitions because of their poor reliability. In particular, pre-wound spring mechanisms fatigue over time, and complex winding or other energizing mechanisms are inherently less reliable.
Although unique pyrotechnic delays that utilize heat transfer through various media to provide a thermal output with a short delay time, i.e., less than about 0.5 seconds, are known in the art, there is no known disclosure of pyrotechnic delays having a non-detonating thermal output that are capable of providing a reliable delay time of greater than 0.5 seconds. For example, U.S. Pat. No. 2,506,157 to Loret discloses a delay action blasting cap that allows a series of blasting caps to be produced having delays that differ one from another by small fractions of a second. The delay action blasting cap comprises an ignition charge in intimate contact with one end of a piece of heat conducting incombustible material, having an explosive fulminating element, i.e., a primary explosive that detonates upon ignition, at the end opposite the ignition charge. Upon combustion of the ignition charge, heat is transferred to, and travels through the piece of heat conducting material. The transfer of heat through the heat conducting material to the fulminating element causes the fulminating element to detonate, resulting in the detonation of the output charge. The amount of time required for the heat to travel from the ignition charge, through the heat conducting material to the fulminating element, causing the fulminating element to detonate, is the delay time of the blasting cap. However, the detonating output of such a delay is not as practical for initiating a propellant or pyrotechnic device that requires a heat and/or flame output.
U.S. Pat. No. 2,429,490 to Scherrer discloses detonators having delay times of from about 5 to about 30 milliseconds (ms). The delay is obtained by placing a thin metal disk, e.g., about 0.0015 inch thick, between a heating charge and a detonating charge. Heat generated by the combustion of the heating charge is rapidly transmitted through the disk to initiate the detonating charge after a short delay.
U.S. Pat. No. 3,727,552 to Zakheim discloses a bidirectional delay connector comprising a shell containing a separate detonating charge adjacent to each end of the connector, where the ends are adapted to receive a detonating fuse. Each detonating charge is also in close proximity to an exothermic charge at an end of a centrally located metal relay capsule, where a heat-conductive metallic delay element is positioned between each heat sensitive charge and the relay capsule containing the exothermic charges. Delay times on the order of 200 ms are produced.
U.S. Pat. No. 3,999,484 to Evans discloses a delay device having a dimpled transfer disc positioned between a delay charge and an output explosive charge. The delay time of 20 ms to 20 s is provided by the burning time of the delay charge. The disc, which is typically only about 0.01 inch thick, contributes little to the overall delay time.
U.S. Pat. Nos. 4,358,998 and 5,593,181 to Schneider et al. and Walker et al., respectively, disclose igniters for pyrotechnic gas bag inflators for vehicles, where short delays on the order of a few milliseconds are required.
Therefore, as a result of a new emphasis on munition self-destruct or disable ("SD"), a need exists for a small, reliable, low cost delay mechanism having a non-detonating thermal output. The present invention provides such a delay.